The first commissioned shoot I ever did was a senior shoot of my friend, so I understand your circumstances completely!
I would start off by doing what DiskoJoe suggested; get to know your friends tastes and preferences. What is the look she is going for? Does she play and sports or instruments that she wants included in some shots? Those are important questions to ask.
One thing that I found to be VERY helpful is to do "pre-shoot homework". Go to this old fashioned down-town district and look around. Find places that would provide great backgrounds and think about different angles and shots you could take at those locations. I like to actually photograph different locations around town so I can remember how they look and "pre-plan" some of my shots. It wastes less time during the actual photoshoot, and makes you look more prepared and professional.
Look for things like interesting siding, brick walls, fire escapes, wooden/stone stairs, peeling paint, old-fashioned windows or door frames, etc. Train tracks are also frequently used in the "senior portrait" realm. In my town, we actually have an old stationary train that alot of kids like to get their senior pictures taken on. Just try to think outside the box. Another thing I have found helpful is to search around on local photographers blogs/websites at their senior sessions; use as inspiration. You can get ideas for locations, poses, etc., but of course don't go to the same locations and do the same poses. Just use as reference and to get ideas.